The present invention relates to a projection type image display system and a projection lens unit used therefor.
A projection lens unit used in a rear projection type image display system have been known as disclosed in, for example, JP-A-1-250916 and JP-A-7-159688, JP-A-9-159914. JP-A-1-250916 discloses a four-group-four lens configuration consisting of a spherical glass lens having high dispersion and a low refractive index as a power lens having a highest positive refractive power, and three aspherical lenses, and JP-A-7-159688 discloses a five-group-five lens configuration consisting of a spherical glass lens having lowe dispersion and a high refractive index as the power lens, and four aspherical lenses while JP-A-9-159914 discloses a six-group-six lens configuration consisting of a spherical glass lens having high dispersion and a low refractive index as the above-mentioned power lens, and five aspherical lenses.
It has been required that the projection lens unit used in a rear projection type image display system has a short focal distance in order to make the lens unit compact, can display an image which is bright in its entirety up to its peripheral edge, and has a satisfactory focusing function and a low cost. It is, in general, a most effective way to use a compact projection lens unit in which the focal distance is short while the number of used lenses is as small as possible in order to make the rear projection type image display system compact, and an inexpensive spherical glass lens is used as a power lens in order to reduce the cost. In general, the higher the refractive index, the higher the cost of the optical glass.
JP-A-1-250916 discloses a power lens made of optical glass having a refractive index of about 1.59 which is relatively low, an Abbe's number of 61.3 and having high dispersion (SK5 (trade name) manufactured by SCHOTT Co., has been typically used as such optical glass). This optical glass is typically used as optical glass for projection type optical lens units. Further, since this optical glass allows the projection lens unit to have a less number of lenses, the cost of the projection lens unit can be reduced. However, since the power lens is made of optical glass having a low refractive index and high dispersion, a desired refractive power cannot be obtained so that the focal distance becomes longer, resulting in difficulty of making the projection lens unit compact. In addition, aberration would be increased, which could be hardly corrected in a satisfactory manner. As a result, although the cost of the lens unit can be reduced, it is difficult to materialize a projection type display unit which is compact and which has satisfactorily corrected aberration.
The JP-A-7-159688 discloses optical glass having a refractive index of about 1.62 and an Abbe's number of 60.3, which has a high refractive index and high dispersion and which is for the above-mentioned power lens (SK16 (trade name) manufactured by SCHOTT Co. has been typically used as such optical glass). The cost of the optical glass is 2.1 times as high as that of the optical glass used in the projection lens unit disclosed in JP-A-1-250916, that is, the former is about two times as high as the latter. However, since the power lens is made of optical glass having a high refractive index and low dispersion, the focal distance can be shortened (the view angle is increased), and the projection type image display system can be made to be compact. Further, the number of spherical lenses for aberration correction is greater than that of the projection lens unit disclosed in JP-A-1-250916 by one so as to make the aberration correction satisfactory. As a result, the projection type image display system can be made to be compact, but the cost thereof can hardly be reduced.
JP-A-9-159914 discloses a power lens made of inexpensive optical glass having a low refractive index and high dispersion, the cost of the projection lens unit can be reduced. However, since the optical glass having a low refractive index and high dispersion, the curvature of the power lens is small so that aberration which would be caused becomes greater. Thus, a number of used aspherical lenses is larger, resulting in a larger number of components although aberration connection is satisfactory is satisfactory, and accordingly, the cost of the projection lens unit can hardly be reduced.
Further, it is required that the projection lens unit has enhanced contrast. In general, the focusing performance of a projection lens unit has been exhibited by only its imaging performance (a degree of aberration correction) of a projection lens. Meanwhile, a contrast ratio, for example, between brightness of a white letter displayed on a screen and brightness of black indication (no image signal) is an important factor for determining a focusing performance in view of an image quality of a rear projection type image display system. In order to enhance the contrast ratio, it is required to prevent light (stray light) from reflecting back to an original image displayed on an image source from lens groups in the projection lens unit.
Thus, it is required that the projection lens unit used in the rear projection type image display system can have a low cost, and can display a projected image having a high quality, being capable of satisfactory aberration correction even though the focal distance thereof is shortened (for broadening its view angle) in order to make the rear projection type image display unit compact.